Implantation requires proper interactions between the blastocyst and the endometrium. Since there are indications that preimplantation embryos may secrete steroid hormones and may locally stimulate the secretory activity of the endometrium, we propose here to examine closely whether the blastocyst can exert some local effects on the uterus during delayed implantation period and during implantation. Specific studies are as follows: (1) The effect of blastocysts on the uterine uptake and distribution of tritiated estradiol-l7beta and amino ACIDS in ovariectomized mice, using embryo transfer and double isotope labeling techniques; (2) The effect of intrauterine instillation of estrogen synthetase (aromatase) inhibitor, 4-hydroxy-4-androstene-3,17-dione or 4-acetoxy-4-androstene-3,17-dione, on implantation in rabbits, for examining the role of embryonic estrogens in implantation. In addition, in vitro conversion of tritiated testosterone to estradiol-17beta by rabbit blastocysts, as affected by aromatase inhibitors, will be studied.